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LDC2114: Issues with coil design

paul Marson
Prodigy 90 points
Part Number: LDC2114

I have a 10mmx8mm coil, with a free air inductance of about 2.6uH and a resistance of ~2ohms. If I connect to the LDC2114EVM board, with its 47p capacitor, I can get reasonable results detecting an aluminium can.  The coils oscillates at 14MHz. But running the calculations, this setup would result in a Q of 118 and an Rp of 28k - well outside the datasheet spec.

If I take the exact same coil, and resonate it with 377pF, it oscillates at 6.6MHz , Q is 31 and Rp is 2k - BUT IT DOESN"T WORK AS A SENSOR!  The data stream just shows a steady output near zero (+/- 1 count), even though I can see the oscillation frequency rise as I bring the aluminium can close to the coil.

Can anyone explain this behaviour?

Note: channel one on this chip doesn't seem to work at all (no oscillation) - could I have a damaged chip that is causing the coil issues?

  • 0
    TI__Genius 11340 points
    Hi Paul,

    It is most likely that some of your register settings regarding sampling window are wrong. For 6.6MHz sensor frequency, in order to achieve a sampling window of 1 ms, you would need to set LCDIV = 1, SENCYCn = 25, and CNTSCn = 0.

    Is your channel 1 turned off? Could you double check the EN1 register to make sure it is enabled?

    Best,
    Jiashow
  • 0 in reply to Jiashow Ho
    Prodigy 90 points

    As far as I can tell all channels are turned on - that's what is showing in the config page of the sensor GUI.

    How exactly is the inductance measurement done? Is the idea to set up a gating signal based on a fixed number of oscillation cycles from a sensor, and use that to gate an internal oscillator and counter to do the conversion? Then the gate signal time would vary with inductance?  It helps to understand the working principle when tweaking all these registers.

    Should we be aiming for 1ms, even though the docs say up to 8ms is ok?

    - Paul.

  • 0 in reply to paul Marson
    TI__Genius 11340 points

    Hi Paul,

    We measured the inductance based on an LC tank oscillation frequency. As a conductive target moves towards the LC tank, the inductor creates magnetic field which causes eddy currents on the target's surface. The eddy currents generate their own magnetic field which opposes the inductor's field, resulting in a reduction in inductance. 

    To set LCDIV, SENCYCn, and CNTSCn registers correctly, you can refer to the equations and chart on section 8.1.6 in the LDC2114 datasheet.

    Typically we recommend ~1 ms of sampling window as higher sampling window only leads to better SNR very slightly.

    Best,

    Jiashow